This invention relates to helical antennas and in particular to improvements in conventional uniform diameter and continuously tapered helix type structures.
Helical antennas are generally constructed with a uniform diameter or a tapered diameter. Although helical gain characteristics over a wide bandwidth for this type of device are not readily available in the literature, extensive testing of particular structures has demonstrated that they exhibit certain deficiencies that limit their usefulness. For example, in one particular application it was necessary to optimize the gain of a helical antenna in the lower portion of a certain band (773 to 1067 MHz) without substantial gain degradation in the upper portion of the band. It was found that a conventional uniform diameter helix was not suitable for applications requiring optimal gain over such a wide band of frequencies.
It has also been determined that uniform diameter helical antennas have large axial ratio (greater than 1 dB over the operating frequency band) and that their beam shape and sidelobe characteristics are often less than satisfactory.
Accordingly, there currently exists the need for broadband helical antennas having wide operating bandwidths, lower axial ratios and better beam shape and sidelobe characteristics than can be achieved with existing devices. It is also desirable that such antennas provide a relatively constant gain over a specified bandwidth. The present invention is directed toward satisfying these needs. The present invention further provides flexibility in the helix design to enable the antenna to meet specified gain-frequency response. With proper choice of diameters and lengths of individual helix sections the antenna can be synthesized to yield a higher (or lower) gain at the low-end of the frequency band or vice versa.